1. Field of the Invention
This invention relates to the field of electric motors and, more particularly, to a motor control system for protecting an electric drive motor against excessive torque when reversing the direction of rotation.
2. Description of the Prior Art
Electric motors generally perform as either generators or alternators when they are driven. A universal type (series wound) electric motor, commonly employed as the drive motor or traction motor in, for example, electric forklift trucks, acts as a generator even when it is operated as a motor. The generated voltage is referred to as back EMF (electromotive force), and serves to oppose supply voltage, thereby decreasing the net voltage available to power the motor. When a series wound electric motor is running in one direction and is then switched into reverse, back EMF and supply voltage become additive, causing the motor to develop a sudden, strong reverse torque, a procedure commonly referred to as "plugging". Plugging is used in some instances by operators as a means of braking the vehicle or motor; however, the sudden torque can damage the motor or other associated elements. For example, with electric forklift trucks, plugging, or the application of a high reverse torque, can damage the gear train of the truck, can throw loads from the load-carrying forks, or can even upset the vehicle under the right conditions. Thus, reversing torque must be either appropriately limited and controlled, or it must be eliminated altogether. Total elimination can easily be accomplished by shutting down the rotation of the motor in one direction and waiting for the motor to stop before reversing the rotation. However, this practice is wasteful in terms of time and energy consumption, and the plugging feature may be desirable in some instances as, for example, an auxiliary braking method, especially in emergency situations or other instances where reversal of the motor must be effected quickly.